Rotors of the type to which the present invention is directed, and machines using such rotors, are well-known and generally include cooperating pairs of rotors to perform work on a fluid. i.e., gas or liquid. A typical rotor pair includes a male rotor and a female rotor. Each rotor has several axially-extending lands that extend along the length of the rotor, and a corresponding number of grooves located between the lands. The radially outermost portion of each land defines the tip of a rotor lobe. The lobes of one rotor engage the grooves of the other rotor, such that the rotors rotate about their respective axes and in directions opposite to one another.
Each rotor includes a profile, which is the shape of the rotor viewed in a plane perpendicular to the axis of the rotor, e.g., a cross-section. The male rotor profile is generally characterized by convex surfaces, while the female rotor profile is generally characterized by concave surfaces. The male lobe profile includes a leading or driving flank, which drives a trailing or driven surface of the female rotor. The male driving and female driven flanks comprise the driving surfaces of a rotor pair.
The efficiency of cooperating screw rotors, and screw rotor machines, depends to a substantial extent upon how well the rotors mesh with one another during operation, i.e., how closely the corresponding rotor surfaces interact as the rotors rotate to one another. For example, as cooperating rotors rotate in opposite directions relative to one another, a tip of one rotor passes through a groove of the other rotor. On one head, if a tip of one rotor does not come into close proximity with the corresponding groove of the other rotor, the fluid disposed between the rotors escapes through the gap between the tip and the groove. One area at which the gap must be minimized is where the male tip and female tip rotate past one another, commonly referred to as a "blow hole". On the other hand, if the tip of one rotor extends too far, the rotors interfere with one another, and cannot rotate.
Those skilled in the art will recognize that more efficient screw rotors are typically defined by grooves that are asymmetric about a line drawn from the center of the rotor through the bottom of a groove. Known asymmetric screw rotors include male and female rotors having profiles that are defined by segments consisting of lines, arcs of circles, ellipses, trochoids, parabolas, involutes, hyperbolas and other fixed geometric shapes. However, rotors of the type characterized by fixed geometric shapes, and machines using these rotors are generally characterized by a limited efficiency and/or mechanical reliability. Moreover, such rotors and machines do not allow for the modification of a rotor profile to minimize clearances between rotors in order to provide increased efficiency and/or mechanical reliability.